Lecture Twelve - Igneous and metamorphic geology Flashcards
On what time scale did the earth differentiate?
Explain the characteristics of the core.
Core:
Comprises 30% of the mass of the Earth, 15% volume.
It consists of a fluid outer core and a solid inner core.
+3400km thick and extends about half way from the centre to the surface.
Formed early in the Earth’s history (first 30 my) but when it ceased to segregate is unknown.
Chemically and physically isolated from the silicate mantle.
Composed of ~ 5/7% Ni, 10% S, less than 3% C, O, P with the rest being metallic Fe - We known this from seismic tomography and refraction, adn from metallic meterites.
We also theorise that the core is a sink for other metallic elements.
Explain the characteristics of the core mantle boundary.
Core/mantle boundary:
A distinct seismic discontinuity present in the upper 200km of the core, or approx. 2900km depth in the Earth.
This layer is very heteroheneous and has a remarkable amount of topography, this is called the D’’.
Observed by seismology - seismic waves reflect and refract dueing earthquakes and atomic testing.
Two schools of though about the large amounts of topography at this boundary:
D’’ is a chemical reaction front between silicates (in the mantle) and metals (in teh core).
OR
It is an accumulation zone of subducted oceanic slabs.
Potentially the origin of some thermal mantle plume magmatism.
Explain the characteristics of the mantle.
Comprises of about 60% of the mass of the Earth, and 84% of its volume.
Lithosphere = crust and upper mantle.
The thickness of the lithosphere varies dramatically and this is mainly because of the two types of crust.
Explain the mantle/crust boundary.
The boundary between the crust and mantle is define by an abrupt change in chemical composition (mantle = olivine rich, crust = feldspar rich).
This compositional change is accompanied by a major change in seismic velocities across this boundary.
This discontinuity is called the Moho -> short for Mohorovicic Discontinuity.
The Moho is commonly at 35-40 km depth beneath the continents but it can be as deep as 70km.
In continents, the Moho is the transitino from quartz rich rocks (i.e. granitic continental crust) to ‘peridotide’ - the main upper mantle material/rock type (rich in olivine and pyroxene).
in the oceanic crust, the Moho is a transition between basaltic oceanic crust and peridotite.
Explain the characteristics of the crust.
Oceanic crust:
Thin ~ 6km.
Nowhere it is older than ~200my due to subduction.
Beneath the oceanic crust is a correspondingly thick oceanic lithosphereic materia.
Continental crust:
35-40km thick on average.
Relitively perminent (the oldest part of the lithosphere - 4.2 Ga dated zircons from Jacks Hill in WA - ‘cratons’).
Beneath the continental crust is a correspondingly thick subcontinental lithospheric material (125-300km) (SCLM).
Where is most of the Fe found in the Earth, why is the crust rich in Si, Al, but depleasted in Fe and Mg?
Most of the Fe is in the core.
Some processes that produce the crust (from melting mantle) are responsible for high % of Si and Al, but low % of Fe and Mg.
What are bulk silicate earth?
The mantle + the crust = the bulk silicate earth (BSE). I.e. the whole earth minus the core.
Is the mantle liquid?
The mantle and crust are not normally liquids - they are solids with varying viscosities.
This means that they flow but are still SOLID.
To start melting them, we must change the temperature and pressure such that the cross the solidus (similar to the melting point).
What will cause the rocks in the mantle to cross their solidus?
Under normal conditions, the mantle geother does not intersect the solidus and will not start to melt.
To generate a melt, we must pass a solid rock sample across its solidus in to the partial melt transition zone.
We never see total melting, more like 30% at most, and more commonly less that 10%.
We refer to the amount of parial melbing by ‘F.’ 10% partial melting, F=0.1.
What happens when there is partial melting in rocks of different minerals?
Because most rocks are made ip of more than one mineral, there will generally be some minerals which melt at different temperatures to one another. Therefore the minerals with lower solidus points will melt before other minerals.
What must you do with temperature and pressure in order to cause melting?
Reduce the pressure;
Adiabatic decompression - pressure released (lower P) melting.
Occurs at spreading centres (MOR) where two pieces of oceanic lithosphere spread apart.
Most common type of melting in the earth - produces new oceanic crust.
Increase the temperature:
Thermal instabilities in the mantle can cause area of localised higher temperature that can melt the surroundings.
This causes mantle plumes that can rise towards the earths surface.
This is expressed as ‘hot-spots’ in oceanic basins and may leave a trail of volcanic islands as the plate moves over the stationary hot-spot. E.g. Hawaii.
Add water:
Water and carbon dioxide when mixed with hot rocks break chemical bonds.
So rock can melt at a lower temperature with addition of water (flux melting).
Lowers the solidus so that it intersects the mantle geotherm.
Occurs in subduction zones as the ‘wet’ subduction rocks dehydrate and lose water to the mantle ‘wedge’ above, causing flux melting.
Subduction zones come in two forms:
1) Oceanic crust oceanic crust convergence -> island arc basaltic (IAB) volcanism.
2) Oceanic crust continental crust convergence -> continental arc volcanism.
